Ultrastructural localization of tyrosine hydroxylase‐like immunoreactivity in the rat hippocampal formation

Abstract

The light and electron microscopic localization of antigenic sites for a polyclonal antiserum directed against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), was examined in the hippocampal formation of the rat brain with a double-bridged peroxidase-antiperoxidase method. By light microscopy, the majority of varicose processes with intense TH-like immunoreactivity (LI) were contained in the hilus of the dentate gyrus (DG) and strata radiatum and lacunosum-moleculare of the CA3 region of the hippocampus. Only a few immunoreactive fibers were observed in the molecular and granule cell layers of the DG, in strata oriens and pyramidale of CA3, and in all layers of CA1. Electron microscopy confirmed that these labeled processes were primarily axons and axon terminals. Terminals with TH-LI were 0.4-1.1 micron in diameter and contained many small clear vesicles and from 0 to 3 larger dense-core vesicles. The number and types of associations formed by terminals with TH-LI were remarkably similar in the DG and hippocampus proper despite known differences in intrinsic cells and function. In both regions, the majority of terminals with TH-LI formed junctions on small (distal dendrites (52% of 112 in the DG; 67% of 116 in CA3) and dendritic spines (30% in the DG; 18% in CA3) that were both asymmetric and symmetric. In the DG, axosomatic junctions (2% of 112) were symmetric and occurred exclusively on the perikarya of granule cells, whereas junctions on large (proximal) dendrites were more numerous (16%), exhibited symmetric as well as asymmetric membrane specializations, and were of both granule (molecular layer) and nongranule (hilus) cell origin. In CA3, synaptic contacts on perikarya (5% of 116) and large (proximal) dendrites (10%) of both pyramidal cell and nonpyramidal cell origin were few and all symmetric. The distribution and types of synaptic associations formed by terminals with TH-LI in the CA1 region paralleled that seen in the CA3 region. In both the dentate and hippocampus proper, 10% of the terminals with TH-LI were observed closely apposed to unlabeled terminals that formed asymmetric synapses with dendrites and dendritic spines. In rare instances, TH-immunoreactive terminals were found in close association with the basement membrane of blood vessels, astrocytic processes, or with other unlabeled terminals not forming recognizable junctions. In addition TH-LI was occasionally detected within the cytoplasm of a minority of astrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

GABAergic neurons in the rat hippocampal formation: ultrastructure and synaptic relationships with catecholaminergic terminals

“…The immunoautoradiographic localization of TH in axons and axon terminals that are primarily in the infragranular zone of the DG and strata lacunosum-moleculare and radiatum of the CA3 region of the hippocampus is consistent with our earlier studies utilizing peroxidase methods (Milner and Bacon, 1989). Additionally, the morphological characteristics of the TH-containing terminals as well as the types and distribution of synaptic contacts on the target neurons were similar to those identified previously by peroxidase labeling methods (Milner and Bacon, 1989).…”

“…Junctions formed by the THlabeled terminals on GABAergic neurons were both asymmetric (Gray type I) and symmetric (Gray type II). The presence of both types of synaptic specializations are similar to our earlier electron microscopic studies of TH-labeled terminals in the hippocampal formation (Milner and Bacon, 1989). Asymmetric synapses are believed to mediate excitation.…”

“…Additionally, the morphological characteristics of the TH-containing terminals as well as the types and distribution of synaptic contacts on the target neurons were similar to those identified previously by peroxidase labeling methods (Milner and Bacon, 1989). The relative distribution of catecholaminergic axons and terminals using TH immunocytochemistry is similar to that described using catecholamine histofluorescence (Loy et al, 1980) or dopamine+hydroxylase immunocytochemistry (Swanson and Hartman, 1975), suggesting that the majority of TH-labeled profiles are noradrenergic.…”

“…The regions of the forebrain containing the hippocampal formation (as described by Paxinos and Watson, 1986) were removed and cut into coronal blocks 4 mm thick and stored in the latter fixative for an additional 30 min. Sections (40 pm thick) were then cut on a Vibratome and collected in 0.1 M phosphate buffer and treated with 1% sodium borohydride prior to immunocytochemical labeling as described previously (Milner and Bacon, 1989). Antisera.…”

“…The possibility for such interactions is supported by the light microscopic detection of GABA-containing neurons and processes in an overlapping distribution with catecholaminergic terminals in the hippocampal formation (Sloviter and Nilaver, 1987;Milner and Bacon, 1989). Moreover, electron microscopic evidence in the CA3 region of the hippocampus (Frotscher and Leranth, 1988) indicates that catecholaminergic terminals synapse on neurons immunoreactive for glutamic acid decarbox-ylase (GAD), the synthetic enzyme for GABA.…”